The present invention relates to a resolver/digital converter, and particularly to a resolver/digital converter having a failure detecting function or to the failure detecting function of the resolver/digital converter.
The servo control system needs a rotation-angle sensor in order to detect the rotation angle and make the feedback control. The brushless motor control also requires the rotation-angle sensor in order to cause current to flow in the coil of the motor in accordance with the rotation angle of the motor as well as the servo control system does. The resolver has so far been widely used as a rotation-angle sensor since it is rigid and resistant to the environment because of its simple structure.
In addition, the servo control system that is used for motor-driven power steering, or x-by-wire control system, particularly steer-by-wire or fly-by-wire control system is required to have a failure detecting function because it needs safety and reliability.
Moreover, a resolver/digital converter has been developed for converting the signal from the resolver to a rotation angle and supplying it as digital data to a microcomputer or the like (see the catalog of Smartcoder AU6802, Tamagawa Seiki Co., T12-159N1 (Apr. 20, 2002), http://www.tamagawa-seiki.co.jp/ctl/1591n3j.pdf (nonpatent literature 1)). The resolver/digital converter according to this document forms a feedback loop that the estimate φ of a rotation angle is modified by the residual error ε resulting from the computation of sin(φ), cos(φ) generated from the estimate φ of rotation angle and the input signal, thereby making the estimate φ of rotation angle be converged to the actual rotation angle θ. This system also has the function to notify the microcomputer of the occurrence of a failure when the residual error ε exceeds a certain value.
In addition, according to JP-A-9-280890 (patent document 1), the failure of the resolver is detected by using the property of trigonometric function that sin2(θ)+cos2(θ)=1 as the output from the resolver.
Furthermore, according to JP-A-9-72758 (patent document 2), an abnormality-detecting signal processor for finding the angle θ is provided independently of an angle-detecting signal processor, and the failure is detected from the difference between the angles θ produced from both processors.